We believe that your issue has been closed by the upload of
Version 1.4.0 of grml-etc from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Hash: SHA1

Format: 1.8
Date: Fri, 22 Jun 2012 23:06:04 +0200
Source: grml-etc
Binary: grml-etc
Architecture: source all
Version: 1.4.0
Distribution: grml-testing
Urgency: low
Maintainer: Michael Prokop <mika@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description: 
 grml-etc   - ecetera files for the grml system
Changes: 
 grml-etc (1.4.0) grml-testing; urgency=low
 .
   * [8d69288] grml-reboot: use virt-what instead of imvirt for detection
     of VMs. Thanks to Ulrich Dangel for bugreport and suggestion
     [Closes: issue1184]
   * [db30a1c] Debian packaging: drop Suggests, Replaces and Recommends
     lines
   * [624ecc3] Bump Standards-Version to 3.9.3.
Checksums-Sha1: 
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 782c7926e96375feae522f7558164ca8707a42a2 107248 grml-etc_1.4.0.tar.gz
 d901d86a48cc12d3c6e8e10bedc6d974b9e965cd 108386 grml-etc_1.4.0_all.deb
Checksums-Sha256: 
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 5b341b8abc0b0f42c93f74f0c170e7d3095a979a6807014234bf77a349645ec1 107248 grml-etc_1.4.0.tar.gz
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Files: 
 6f932e0b2de72815e2296be522702025 899 grml optional grml-etc_1.4.0.dsc
 9c1555ba6cc74d24b3857a641e47f084 107248 grml optional grml-etc_1.4.0.tar.gz
 1799e6ddcaf1725d9e6eb5d345e94ae6 108386 grml optional grml-etc_1.4.0_all.deb

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While running the Grml 2012.05 in Virtualbox i discovered that imvirt returns 
physical for the virtual machine. This means at shutdown grml waits until i press 
enter (noprompt).

I suggest to use virt-what instead of imvirt as virt-what returns virtualbox